Metal-to-ceramic seal for high voltage electron tubes and methods of fabrication



July 30, 1968 w. R. STUART 3,394,451

METAL-TO-CERAMIC SEAL FOR HIGH VOLTAGE ELECTRON TUBES AND METHODS OF FABRICATIQN Filed July 28, 1965 I NVEN TOR. WILLIAM R. STUART ATTORNEY wow Pat 3 3,394,451 METAL-TO-CERAMIC SEAL FOR HIGH VOLT- AGE ELECTRON TUBES AND METHODS OF FABRICATION a William R. Stuart, San Carlos, Califi, assignor,--by mesne assignments, to Varian Associates, a corporationof California Filed July,28, 1965, Ser. No. 475,527

8.Claim s. (Cl. 29473.1)

ABSTRACT OF THE DISCLOSURE Method of producing a vacuum-tight seal between a ceramic body and a metal body in an electron tube. A selectedseal'area on the ceramic is first metallized. The metal body is next brazedto the ceramic body at the seal area. Metallizingradjacent the seal area is then mechanicallyremoved to inhibit electric discharge from the seal.

This invention relates tovacuum-tight metal-torc ramic seal structures and more specifically to vacuum-tight metal-to-ceramic structures designed to minimize undesirable effects of high voltage applied thereto.

It has been found that in certain vacuum-tight metalto-cerarnic structures, which include a ceramic member sealed between two metal elements, a threshold voltage condition between the two metal elements can be reached whereby an apparent electric discharge is generated which fans out from point sources of ceramic metalizing material -deposited along the interior, or vacuum side, of the ceramic member. Thismetalizing material is deposited when the ceramic member is being metalized in preparation for making a metal-to-ceramic braze seal.

In the particular case investigated involving a small tetrode (250-watt plate dissipation) the electric discharge occurred from apparent point sources adjacent the screen grid-ceramic seal and fanned out across the ceramic cylinder towards the tubes anode. The electric, discharge, resulting from a low impedance anode-to-screen grid path, caused excessive plate currentto flow. A fast blowing fuse in the anode circuit would blow out whenever the discharge occurred. It has been found that this phenomena may be eliminated by removing the metalizing material at the internal negative potential end of the ceramic cylinder.

One process found completely effective is to grind a bevel onto the inside metalized area of what will be the negative potential end of the ceramic cylinder of the tetrode prior to brazing the metal-to-ceramic seal. This beveling is done after the metalizing has been completely processed on the ceramic and the operation removes both the non-desired metalizing material and the ceramic underbase immediately therebelow. This grinding operation, however, was found to be expensive for normal high volume manufacturing processes. Another, less expensive, process has been found for removing the non-desired metalized material from the interior of the ceramic cylinder whereby said non-desired metalizing material is removed by an air-driven abrasive, or sand-blasting, prior to firing of said metalizing. This process does not necessarily remove any ceramic underbase.

An object of this invention is to provide a metal-toceramic seal which is specifically adapted for high voltage application.

This and other objects are accomplished in accordance with the present invention, by specially processing the ceramic portion of a composite metal-to-ceramic seal prior to assembly.

This invention as well as other objects, features, and advantages thereof, will readily be apparent from consideration of the following detailed description relating to the annexed drawing in which:

FIGURE 1 illustrates, in partial cross-section, an electron tube incorporating a metal-to-ceramic seal;

FIGURE 2 is a detailed drawing, not to scale, of the prior art design of the subject seal, the drawing shows exaggerated metallizing, plating and braze material layers;

FIGURE 3 is a detailed drawing, not to scale, of the subject seal incorporating the beveling removal process of the subject incention, the drawing shows exaggerated metalizing, plating and braze material layers; and

FIGURE 4 is a detailed drawing, not to scale, of the subject seal incorporating the sand-blasting removal process of the subject invention, the drawing shows exaggerated metalizing, plating and braze material layers.

Referring now to the drawings, wherein like reference c haracter'srepresent like or corresponding parts throughout the several views, there is illustrated in FIGURE 1 an electron tube comprised of a header section 10 and an anode section 11.

The header section 10 of the tube is basically a series of concentric cylindrical structures with, shown externally only in FIGURE 1, an outer filament contact 12 and a control grid contact 14 and, shown in partial cross section, a header ceramic support flange 16, header ceramic 18 and header assembly sealing flange 20. The header assembly sealing (flange 20 is actually an arcuate extension of the header ceramic support flange 16. The header ceramic support flange 16 is hermetically sealed to the header ceramic 18 with a metal-to-ceramic seal 22.

The anode section 11 of the tube is also basically a series of concentric cylindrical structures with, shown externally only in FIGURE 1, a tulbulation cap 30, a tubulation cap flange 32, and a composite cooling fin structure 34, and shown in partial cross-section, an anode core 36, an anode core sealing flange 38, a ceramic cylinder 40 and a screen grid contact flange 42. The anode core 36 is brazed 44 to the anode core sealing flange 38. The anode core sealing flange 38 is hermetically sealed to the ceramic cylinder 40 with a metal-to-ceramic seal 46. The ceramic cylinder 40 is hermetically sealed to the screen grid contact flange 42 with a hermetic metal-toceramic seal 50 incorporating an embodiment of the invention.

FIGURE 2 is a detailed non-scale cross-sectional drawing of the prior art metal-to-ceramic seal 50. In the manufacture of seal 50' metalizing material 52' is painted, or otherwise deposited, on the end face 54 of the ceramic cylinder 40'. Some of the metalizing material 52' runs, or is otherwise deposited, onto the ceramic surfaces adjacent the end face 54'. The metalizing material 52' is then sinter fired and plated with a secondary metal 56' such as copper or nickel. The secondary metal 56' is next sinterfired. A hermetic braze joint is then made between the sinter-fired plating 56' and the screen grid contact flange 42' with a layer of braze material 58' therebetween.

In particularly high voltage applications involving the prior art metal-to-ceramic seal (FIGURE 2), the apparent electric discharge occurs within the electron tube in the general area 59' where the metalizing material 52' extends around the end face 54 and along the inside surface 60 of the ceramic cylinder 40-. The electric discharge appears to emit from very small point sources of metalizing material 52' and fan out across the inside surface 60' of the ceramic cylinder 40' in the direction generally away from the screen grid contact flange 42.

FIGURE 3 is a detailed non-scale cross-sectional drawing of an improved metal-to-ceralmic seal 50 wherein the metalizing material 52" is applied on the end face 54" of the cylinder 40" and sinterfired as in the prior metalizing material 52", the inside, or vacuum side 60? of the end face of the ceramic cylinder is ground off 62" as well as any covering of metalizing material thereon. The remaining sinter-firedmetalizing material 52" is then plated with a secondary metal 56" and the secondary metal 56 is then, too, sinter-fired. A hermetic braze joint is then formed between the sinter-fired plating 56" and the screen grid contact tfiange 42", with a layer of braze material 58" therebetween. A 'henmetic metal-to-ceramic seal 50 is thus formed.

FIGURE 4 is a detailed non-scale cross-section drawing of another improved metal-to-ceramic seal 50" wherein the mealizing material 52" is applied on the end of face 54" of the ceramic cylinder 40" as in the prior art. However, prior to sinter-firing of the metalizing material 52", the inside, or vacuum side 60", of the end face 54" of the ceramic cylinder 40" is sand-blasted whereby all metalizing material deposited thereon is removed. The remaining metalizing material 52" on both the end face 54 and the adjacent outside wall 64 of the ceramic cylinder 40 is sinter-fired and plated with a secondary metal 56". The secondary metal 56" is then sinter-fired. A hermetic braze joint is then formed between the sinter-fired plating 56" and the screen grid contact flange 42" with a layer of braze material 58" therebetween. A hermetic metal-to-ceramic seal 50" is thus formed.

In the two methods described for fabricating metal-toceramic seals wherein the metalizing material 52 is mechanically removed from the inside surface 60 of the ceramic cylinder 40, the step for removing the metalizing material 52 was specifically noted. These removal steps, however, are not critical in sequence and may be performed, in either case, at any time prior to sealing (with the braze material 58) of the sintered metallic plating 56 and the screen grid contact flange 42. It is easier, however, in the sand-blasting operation to remove the metalizing material 52" prior to sintering as a softer sand may be used.

Chemical removal (mixture of hydrofluoric and nitric acid solutions) of the undesired metalizing material 52 was considered, prior to adoption of the mechanical removal procedures, but was found to be unsatisfactory as the process was slow, dangerous to the technician doing the work, difiicult to control the actual area cleaned, and detrimental to the strength of the ceramic in that the glassy phase is weakened.

In any fabricating of the improved metal-to-ceramic seal, all metalizing material on the interior surface of the ceramic cylinder must be removed or the aforementioned apparent electric discharge will not be eliminated. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to' be understood, that within the .scope of the appended claims, the invention maybe prac:

ticed otherwise than specifically described.

I claim:

1. In the method of producing a vacuum-tight seal between a ceramic body and a metal body in an electron tube comprising the steps of metalizing a selected seal area on said ceramic body and brazing said ceramic body to said metal body at said seal area, the additional step of mechanically removing all metalizing from said ceramic body adjacent said seal area.

2. The combination in claim 1 wherein said mechanical removal of all metalizing from said ceramic body adjacent said seal area is accomplished by sand-blasting.

3. The combination in claim 1 wherein said mechanical removal of all metalizing from said ceramic body adjacent said seal area is accomplished by grinding of the ceramic surface.

4. The method of producing a metal-to-ceramic hermetic seal in an electron tube comprising: depositing metalizing compound on a selected area of said ceramic, allowing said metalizing compound to dry, mechanically removing all of said metalizing compound deposited adjacent said selected area, sinter-firing said metalizing compound, depositing a secondary metal layer on said metalizing compound, sinter-firing said secondary metal layer, and brazing said secondary metal layer to said metal of said metal-to-ceramic seal.

5. The combination in claim 4 wherein said mechanical removal of said metalizing compound deposited adjacent said selected area is accomplished by sand-blasting.

6. The combination in claim 4 wherein said mechanical removal of said metalizing compound deposited adjacent said selected area is accomplished by grinding of the ceramic surface.

7. The combination in claim 4 wherein said secondary metal layer on said metalizing compound is copper.

8. The combination in claim 4 wherein said secondary metal layer on said metalizing compound is nickel.

References Cited UNITED STATES PATENTS CHARLIE T. MOON,

J. L. CLINE, Assistant Examiner.

Primary Examiner. 

